1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device. In particular, the invention relates to a semiconductor device having an insulated gate transistor and a method of manufacturing the semiconductor device.
2. Description of the Background Art
Recently, in the fields of household electrical appliances and industrial power units for example, inverter devices have become employed. For an inverter device, usually a commercial power supply (AC power supply) is used. Therefore, the inverter device is configured to include a converter unit making a forward conversion of once converting an AC voltage from the AC power supply into a DC voltage, a smoothing circuit and an inverter unit making a reverse conversion of converting the DC voltage into an AC voltage. As a main power element of the inverter unit, an insulated gate bipolar transistor (hereinafter referred to as IGBT) capable of performing relatively high-speed switching operation is chiefly employed.
Regarding an inverter device used for electric-power control, the electric current rating of one IGBT chip is approximately a few tens to a few hundreds of amperes (A) and the voltage rating thereof is approximately a few hundreds to a few thousands of volts (V). In a circuit using an IGBT as a resistance load and successively changing the gate voltage, electric power that is the product of the electric current and voltage is generated in the form of heat within the IGBT. Therefore, the inverter device requires a large heat radiator, resulting in deterioration in electric-power conversion efficiency. Further, depending on the combination of an operating voltage and an operating current, the temperature of the IGBT itself increases, resulting in thermal breakage of the IGBT. As such, in the inverter device, a resistance load circuit using an IGBT as a resistance load is rarely used.
In most of inverter devices, the load is an electric induction machine (inductive load motor). Therefore, in the inverter device, an IGBT is usually operated as a switch to repeat the OFF state and the ON state so as to control the electric power energy. Regarding switching of the inverter circuit of the inductive load, the ON state is established after a turn-on process while the OFF state is established after a turn-off process. The turn-on process refers to transition of the IGBT from the OFF state to the ON state and the turn-off process refers to transition of the IGBT from the ON state to the OFF state.
The inductive load is connected to a point of an intermediate potential between an upper arm element and a lower arm element, and an electric current is flown to the inductive load in both of the positive and negative directions. Therefore, in order to allow the electric current flowing through the inductive load to return from the load-connected portion toward a high-potential power supply or to flow from the load-connected portion toward the ground, a freewheel diode is necessary for circulating the current in a closed circuit of the inductive load and the arm element. For an inverter device having a relatively small capacity, a field-effect transistor (MOSFET: Metal Oxide Semiconductor Field-Effect Transistor) is used in some cases.
A voltage to be applied to the gate electrode for turning on the IGBT is called ON voltage (Vce (sat)). A configuration for providing a lower ON voltage is disclosed, for example, in Japanese Patent Laying-Open Nos. 8-316479 and 2002-353456 disclosing a carrier storage type IGBT which is an improved version of the trench gate type IGBT. In the carrier storage type IGBT, an n-type carrier storage layer is formed on one surface of an N− substrate, and a p-type base region is formed on the carrier storage layer.
In a predetermined region of a surface of the base region, an emitter region is formed. In a region except for the emitter region of the surface of the base region, a p+ contact region is formed. Further, an emitter electrode electrically connected to the emitter region is formed. Through the emitter region, base region and carrier storage layer, an opening is formed to reach an n-type region of the N− substrate, and a gate electrode is formed on the inner wall surface of the opening with a gate insulating film interposed therebetween.
On the other surface of the substrate, an n-type buffer layer is formed, and a p-type collector layer is formed on the buffer layer. On a surface of the collector layer, a collector electrode electrically connected to the collector layer is formed. The conventional carrier storage type IGBT is configured in this way.
In this carrier storage type IGBT, a voltage of at least the threshold voltage (Vth) is applied as a gate voltage to the gate electrode to form an n-channel region that is located near the gate electrode in the p-type base region. Accordingly, electrons are injected from the emitter region through the n-channel region into the N− substrate.
It is supposed that, in the state where a voltage of at least the threshold voltage is applied to the gate electrode, a voltage (collector voltage) is applied to the collector. Under this condition, the collector voltage is applied at least to the extent that causes the pn junction of the buffer layer and the collector layer to be forward biased. Then, from the collector electrode, holes are injected into the N− substrate. In the N− substrate, conductivity modulation occurs to cause the resistance value of the N− substrate to suddenly decrease, and accordingly the electric current flows and the electrically conductive ON state is established.
Thus, in the carrier storage type IGBT, the carrier storage layer located immediately under the p-type base region stores holes and electrons. Therefore, as compared with the trench gate type IGBT without the carrier storage layer, the carrier storage type IGBT has the advantage that the N− substrate has a higher carrier density and accordingly a lower resistivity, and thus a lower ON voltage is achieved.
In recent years, with the purpose of providing a compact and lightweight inverter device, an IGBT has been proposed that is called reverse conducting IGBT or reverse conducting carrier storage type IGBT having a free-wheeling diode formed in the configuration of the IGBT or carrier storage type IGBT and having electrical conduction capability to substantially the same degree in both directions.
The conventional carrier storage type IGBT, however, has the following problem. The n-type emitter region, the p-type base region and the n-type carrier storage layer of the carrier storage type IGBT are each formed by injecting impurity ions of a predetermined conductivity type and thermally diffusing the injected impurities. In the conventional carrier storage type IGBT, respective impurity concentration profiles are each the Gaussian distribution where the maximum impurity concentration is located near the surface of the N− substrate due to limitations of the manufacturing apparatus (ion injection apparatus).
The final impurity concentration profile of the n-type emitter region, the p-type base region and the n-type carrier storage layer is a triple diffusion profile comprised of the three Gaussian-distribution impurity concentration profiles overlapping each other. The triple diffusion profile has a p-type impurity concentration profile or an n-type impurity concentration profile depending on the relative relation or subtraction between respective numbers of impurity atoms. Thus, the threshold voltage (Vth) of the IGBT is likely to vary, as described below.
For a carrier storage type IGBT used for an inverter device, the threshold voltage (Vth) is set to approximately 5 V. Therefore, the maximum concentration of the p-type impurity in a region where the channel is formed along the gate insulating film in the p-type base region is approximately 1×1017 cm−3 to 1×1018 cm−3. In a region corresponding to the p-type base region, for example, the acceptor impurity concentration is at least approximately 1×1018 cm−3 and the donor impurity concentration is approximately 5×1017 cm−3.
The impurity concentration (density) of the portion which is located in the p-type base region and in which the n-type channel is formed is determined by subtracting the number (density) of donor atoms from the number (density) of acceptor atoms (the number of acceptor atoms—the number of donor atoms). The acceptor may be for example boron (B) or aluminum (Al) in silicon (Si), and the donor may be for example phosphorous (P) or arsenic (As) in silicon (Si).
In the case where the accepter impurity concentration and the donor impurity concentration are each on the above-described order, the impurity concentration of the finally formed p-type impurity region is approximately 1×1017 to 2×1017 cm−3. Therefore, the p-type base region has the number (density) of acceptor atoms and the number (density) of donor atoms that are each larger (higher) than the impurity concentration (density) of the p-type base region.
Therefore, in the process of injecting impurity ions acting as acceptors, if the amount of injected impurity ions varies, the impurity concentration of the finally formed p-type base region also varies. In the process of injecting impurity ions acting as donors, if the amount of injected impurity ions varies, the impurity concentration of the p-type base region also varies. In other words, the impurity concentration of the p-type base region is influenced by the variation of the amount of injected impurity ions acting as acceptors and the variation of the amount of injected impurity ions acting as donors.
The threshold voltage of the IGBT has a certain range with respect to the center of variation of the amount of injected impurities. Regarding the conventional IGBT, the standard deviation is large, resulting in variation of the threshold voltage. If the threshold voltage varies and the voltage value is lower than a predetermined voltage, the resultant problem is that the semiconductor device is broken in the load short-circuit operation mode.